Feeler Gauge

A feeler gauge is a tool used to measure the width of a gap, most often the clearance between two parts in an engineering or mechanical context. It consists of a set of small blades of steel, each of a different, precisely known thickness, marked with its own measurement; because the blades are thin and flexible, several can be stacked together to measure a gap that falls between two of the set's standard sizes. Feeler gauge sets are made to imperial standards, in thousandths of an inch, or to metric standards, in hundredths of a millimeter, and some sets include one blade made of brass rather than steel, a holdover from early electronic ignition systems that required the air gap between the reluctor and the pickup to be set with a non-ferrous metal so as not to affect the magnetic reading.